1. Field of the Invention
This invention is related to crystal growth processes in general and to the growth of large single crystals of lead tin telluride in particular.
2. Description of the Prior Art
The phase diagram of lead tin telluride (FIGS. 1 and 2) indicates a narrow separation of the liquidus and solidus curves, thereby enabling the advantageous use of several prior art crystal growth methods. Such methods include the Bridgman-Stockbarger, Czochralski, vapor transport. All these methods have been relatively successful in producing bulk single crystals of lead tin telluride; however, they are deficient in one or more respects.
In the Bridgman-Stockbarger and Czochralski methods, growth proceeds from a melt to a solid. Because the liquidus-solidus curves for lead tin telluride are narrowly separated, the composition of a growing crystal differs from that of the melt from which it grows. Therefore, the resulting crystal does not have a uniform composition but varies, as will be more fully explained with reference to FIGS. 1 and 2.
These prior art techniques further give rise to defects and inhomogeneity as a result of constitutional supercooling. If a proper temperature coolant is not maintained at the solid liquid interface, precipitation of a tin rich phase takes place and gives rise to an undesirable cellular structure.
Other problems arise because these techniques require operation at high temperature in order to obtain the melt. Such high temperatures promote a greater likelihood that impurities will be leached, in particular from the crucible, especially in view of the large contact area between the crucible and the crystal. In addition, these methods require relatively elaborate and expensive equipment.
In the vapor transport method, the source, having the desired composition, is placed in a temperature gradient for sublimation and condensation on a colder surface. Because growth is initiated by spontaneous nucleation, success depends on the ability to obtain the smallest number of nucleation sites, the control of which is very difficult. Thus, this method usually results in the formation of many small points of nucleation at the tip of the tube and their eventual growth together to produce a crystal which is not a single crystal.
The present invention overcomes these and other problems by recognizing that compositional deviation and supercooling problems are avoided by growing a crystal at a constant, low temperature in an environment which minimizes contact between the forming single crystals and the crystal growth tube. Constitutional supercooling is absent since the growth is under near-equilibrium conditions.